I'm thinking something with greens in it perhaps. I have a favorite way of preparing Swiss Chard (http://www.epicurious.com...) also with Kalamatas but I wonder if that would be too dry in a tamale.
I doubt it would be too dry, but depending on the them of the party etc, it might not be what the vegetarians at the party want to eat. If it's a mexican themed dinner I would try to keep the flavors at least somewhat within that palate. These Squash, black bean and goat cheese ones sound pretty good!
or these bean-y ones from The Kitchn, which are actually vegan :
Spinach ,cheese with tomate and cilantro. mmm.
Mushroom and rice
I'm partial to jalapeno and cream cheese tamales
These are all making me hungry! I'll have to try a few of these fillings on Friday when we make the tamales.
My all-time favorite tamale filling is braised bitter greens. I first ate that kind of tamale (actually, a tamalito) from a seller in the Xochimilco market. His were filled with quelites, greens that are indigenous to that area. If I were making tamales, I'd use a combination of Swiss Chard and broccoli rabe to approximate the flavor.
We have a close friend from Guatemala, and for many years, every Christmas Eve she makes tamales for all her friends. Really a labor of love.
They're different than Mexican - wrapped in plantain leaves, not corn husks. One type she always makes are filled with chicken/peppers in an ancho-ish red sauce... but the other is filled with green olives, raisins, nuts, spices, etc. Very traditional from what I gather, so there must be recipes online. Not overly sweet or dessert-like. Seems like even if there were stock in the traditional sauce, you could adapt it to be vegetarian.
My favorite is chiles en nogada. The version I do is roasted poblanos stuffed with marinated portabellos and cashew ricotta, breadcrumb-coated and baked. Then you smother them in a creamy walnut sauce.
(Not tamales, but still Mexican.)